The present invention relates to a cathode ray tube and more specifically to a cathode ray tube which uses an electron gun having a main lens with reduced aberration to improve resolution of images displayed on a direct-view or projection television set or on a display device of information terminal equipment.
Cathode ray tubes are widely used as a display device for direct-view TV, projection TV and information terminal equipment.
This kind of cathode ray tube includes in a vacuum envelope at least an electron gun that emits electron beams and a phosphor screen applied with a phosphor material that illuminates when excited by the bombardment of electrons. The resolution of a displayed image depends on the size and shape of a spot of the electron beam on the phosphor screen.
Because the size and shape of the electron beam spot on the phosphor screen depend on the focusing characteristics of the electron gun, the arrangement and shape of electrodes constituting a lens of the electron gun are extremely important in improving the resolution.
FIG. 1 is a schematic cross-sectional view of a color cathode ray tube showing one example structure of a conventional cathode ray tube and its electron gun. Denoted 1 is a glass envelope, which is a vacuum vessel; 2 a faceplate panel that provides a display screen; 3 a phosphor screen, which is a layer of three-color phosphor material formed over the inner surface of the faceplate panel 2; 4 a shadow mask that forms a color selection electrode; 5 an inner conductive film to be applied with an anode voltage; and 6 an external magnetic deflection yoke. Reference numerals 7, 8 and 9 represent cathodes; 10 a first grid electrode (G1 electrode); 20 a second grid electrode (G2 electrode); 30 a third grid electrode (G3 electrode); 40 a fourth grid electrode (G4 electrode); and 50 a shield cup. The third grid electrode 30 has electron beam passage openings 31, 32 and 33 formed on the side of the fourth grid electrode. The fourth grid electrode 40 has electron beam passage openings 41, 42 and 43 formed on the side of the third grid electrode. The shield cup 50 has electron beam passage openings 51, 52 and 53. Designated 61, 62 and 63 are center axes of three electron beams arranged almost parallel to each other on a horizontal plane. Denoted 64 and 65 are center axes of the side electron beam passage openings 41, 43 of the fourth grid electrode 40 on the third grid electrode side.
In the figure, the cathodes 7, 8, 9, the first grid electrode 10, the second grid electrode 20, the third grid electrode 30, the fourth grid electrode 40, and the shield cup 50 combine to form an electron gun, with the cathodes 7, 8, 9, the first grid electrode 10 and the second grid electrode 20 forming a beam generator section.
The beam generator section emits electron beams along the center axes 61, 62, 63 arranged on a horizontal plane. The electron beams enter a main lens section formed in a space between the third grid electrode 30 and the fourth grid electrode 40.
The main lens section is formed by the third and fourth electrodes 30, 40, which are the main lens electrodes, and the shield cup 50. The center axes of the electron beam passage openings formed in the third electrode 30' one of the two main lens electrodes, and in the shield cup 50 are aligned with the corresponding center axes 61, 62, 63 of the electron beams. As to the electron beam passage openings formed in the fourth electrode 40' the other of the two main lens electrodes, on the third electrode side, the center axis of the center electron beam passage opening 42 coincide with the center axis 62 of the center beam, whereas the center axes 64, 65 of the side electron beam passage openings 41, 43 are not in line with the beam axes 61, 63, respectively and are slightly shifted outwardly.
The potential of the third grid electrode 30 is set lower than that of the fourth grid electrode 40. The higher-potential fourth grid electrode 40 is set equal in potential to the shield cup 50 and the inner conductive film 5.
Since the center electron beam passage openings 32 and 42 of the third and fourth grids 30, 40 are coaxial with each other, an axially symmetrical main lens is formed at the central portion between the third and fourth grid electrodes 30 and 40. The center electron beam is focused by this main lens and advances straight along the center axis 62.
On the other hand, the side electron beam passage openings 31, 33 of the third grid electrode 30 and the outer electron beam passage openings 41, 43 of the fourth grid electrode 40 are out of alignment with each other in their center axes, so that they form axially non-symmetrical main lenses for side electron beams.
In diverging lens regions formed on the fourth grid electrode 40 side of the main lens regions, the side electron beams therefore pass through each lens at a point deviated from the lens center axis toward the center electron beam, so that they are subjected to the focusing action of the main lenses and also to a force urging the beams toward the center beam.
In this way, the three electron beams are focused and at the same time converged so that they overlap each other on the shadow mask 4. The operation for converging these electron beams is referred to as a static convergence.
The electron beams are color-selected by the shadow mask 4 so that only a part of each electron beam that will excite a phosphor area of the corresponding color passes through apertures of the shadow mask 4 and strikes the phosphor screen 3. A deflection magnetic field generated by the external magnetic deflection yoke 6 causes the electron beams to perform a two-dimensional scan on the phosphor screen 3.
The spherical aberration of the main lens is generally known to be a major factor that has a great effect on the resolution characteristic of the aforementioned color cathode ray tube. Reducing the spherical aberration of the main lens will reduce deterioration in the resolution caused by an increase in the spot diameter on the phosphor screen.
It is also known that increasing the diameters of apertures in the electrodes making up the main lens is effective for reducing the spherical aberration of the main lens.
In the in-line type electron gun shown in FIG. 1, however, since the three main lenses of cylinder type corresponding to the three primary colors, red (R), green (G) and blue (B), are arranged in the same plane, the diameters of apertures in the electrodes must be less than one third of the inner diameter of the neck portion of the glass envelope 1. Considering the thickness of electrodes and also problems with electrode manufacturing, the upper limits of diameters of apertures in the electrodes will further decrease. Increasing the inner diameter of the neck portion for the purpose of raising this limit will give rise to another drawback of an increase in the deflection power of the deflection yoke 6 arranged on the outside of the neck portion.
One example of a non-cylindrical main lens whose diameter can be increased virtually beyond the above limit is disclosed in the Japanese Patent Laid-Open No. 103752/1983.
FIG. 2 (a) and FIG. 2 (b) are cross sections showing essential portions of an electron gun having the non-cylindrical lens disclosed in the above-mentioned publication. FIG. 2(a) is a longitudinal cross section and FIG. 2 (b) is a transverse cross section taken along the line 2b--2b of FIG. 2(a). In these figures, designated 300 is a third grid electrode, 400 a fourth grid electrode, 500 a shield cup, 310 a first electrode plate, 311, 312 and 313 electron beam passage openings in the first electrode plate, 410 a second electrode plate, and 411, 412 and 413 electron beam passage openings in the second electrode plate.
As shown in FIG. 2(a), the first electrode plate 310 and the second electrode plate 410 installed at the facing surfaces of the third grid electrode 300 and the fourth grid electrode 400 are set back away from the facing surfaces. As a result, the electric field can penetrate deep into the interior of the third grid electrode 300 and the fourth grid electrode 400, producing the same effect as that obtained when the lens diameter is increased.
The peripheral portion of the cross section of the grid electrodes, however, is not axially symmetrical and its horizontal length in a direction H--H is longer than the vertical length in a direction V--V. Hence, the penetration of the electric field is conspicuously large in the horizontal direction so that the lens focusing force in the horizontal direction becomes weaker than in the vertical direction. This gives rise to an astigmatism, which causes the cross-sectional shape of the electron beam to flatten horizontally, resulting in a lowered resolution.
To correct this problem, the electron beam passage openings 311,312, 313 formed in the first electrode 310 and the electron beam passage openings 411,412, 413 formed in the second electrode 410 are made non-circular in cross section in such a way that their horizontal diameters are smaller than their vertical diameters. This strengthens the horizontal focusing field, balancing the horizontal and vertical focusing forces, and eliminating the astigmatism.
With the conventional electrodes of the shape mentioned above, however, it is not possible to take full advantage of the effect of the enlarged lens diameter for the following reason.
To make full use of the effect of the enlarged lens diameter, it is necessary to expand the electron beam diameter in the main lens. Enlarging the electron beam diameter beyond a certain level, however, results in a part of the electron beam striking the first electrode plate 310. These striking electrons flow into the third grid electrode 300 generating a current. This current then flows to a power supply circuit that generates a focusing voltage, i.e. a voltage to be applied to the third grid electrode 300. Generally, the impedance of the power supply circuit is significantly large, so that the current reduces the output voltage or focusing voltage, making it impossible to operate the electron gun under normal focusing conditions.
Under these situations, it is conceivable to have a structure which eliminates the first electrode plate 310 and the second electrode plate 410. However, removing the first electrode plate 310 and the second electrode plate 410 results in an astigmatism and a significant reduction in the resolution because the peripheral portion of the grid electrode cross section is similar to the shape of a racetrack.
To correct the astigmatism the following means may be conceived. That is, attention is paid to the fact that only the third grid electrode 300 acts to horizontally elongate the electron beam and that the horizontally long cross section of the fourth grid electrode 400, on the contrary, acts to cause the electron beam to become vertically long in cross section. If the out-of-roundness of the peripheral portions of the third grid electrode 300 and of the fourth grid electrode 400 are equal, the electron beam is strongly influenced by the third grid electrode 300 because the electron beam travels slower in the low-voltage third grid electrode 300 and stays there longer. The overall result is that the electron beam is deformed horizontally long, causing the above-mentioned astigmatism.